There are many contexts in which a substantially flat substrate, such as a sheet of paper, is desired to be moved at a precise velocity and direction. A typical context is in printing, either of the digital or traditional types. In the xerographic context, for example, the sheet must move at a precisely-determined velocity to contact a developed image at a photoreceptor, to receive the image at a precise location thereon. Also, the sheet must not be skewed, or otherwise laterally displaced along a main process direction, so the received image is not skewed or improperly placed on the final print.
At high levels of precision, as would be required in a high-speed printing apparatus, the velocity of a sheet moving through a machine cannot be directly assumed by monitoring the motion of parts within the machine, such as rollers which contact and impart motion to the sheet at various times. Even slightly deformable rolls, for instance, do not have an assumable circumference by which the velocity of a sheet in contact therewith can be calculated. Also, brushless DC motors, as are often used in printing machines, are often incapable of operating at sufficiently precise rotational speeds.
In most common systems for monitoring the speed of a sheet passing through a machine, a lead edge of the moving sheet is used, in one of various ways, to interact with a monitoring device, such as for example measuring when the lead edge breaks one or more light beams as it moves. One problem common to such a system is taking into account the skew or other displacement of the sheet relative to an expected path through the machine.
In the prior art, U.S. Pat. No. 5,557,396 discloses a system for using a measured Doppler shift of light reflected from a moving sheet, in order to measure the velocity of the sheet. U.S. Pat. No. 6,741,335 discloses another system for determining the speed of a moving sheet. U.S. Pat. No. 6,533,268 discloses a system that contacts a moving sheet to obtain a desired lateral registration and deskewing.